Pneumatic tyre

ABSTRACT

A pneumatic tyre for a car having a tread, including a central portion disposed astride an equatorial plane and two shoulder portions. The central portion is separated from the tread shoulder portions by two circumferential grooves and at least one circumferential row, included between a first and a second circumferential groove, is present in the central portion. The tread has a hollow/solid ratio lower than 0.28. The circumferential row comprises transverse grooves extending over at least 80% of the width of the circumferential row. Each transverse groove includes a median line having at least one first rectilinear stretch and one curvilinear stretch. The transverse grooves have a smaller width than that of the circumferential grooves.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national phase application based onPCT/IT2007/000774, filed Nov. 5, 2007, the content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a pneumatic tyre for a car. Inparticular a pneumatic tyre for High Performance cars and SUV's (SportUtility Vehicles).

2. Description of the Related Art

Optimal characteristics in terms of traction and braking, as well ashandling on dry and wet road surfaces are usually required from tyres ofthis class, in addition to a satisfactory resistance to wear.

Tyres for motor-vehicles are known which have a tread provided withblocks delimited by circumferential grooves extending in a substantiallylongitudinal direction, and transverse grooves extending in asubstantially axial direction. The blocks resulting from interlacing ofthese grooves are shaped according to different conformations suitablystudied and are disposed in circumferential side-by-side rows each ofwhich is included between two successive circumferential grooves.

The circumferential grooves can affect the features concerning thetyre's steering property and steadiness on running in relation to thelateral (slip) thrusts directed parallel to the rotation axis of thetyre.

The transverse grooves, in turn, can affect the traction abilityfeatures of the tyre, i.e. the capability of transmitting to the roadsurface the tangential thrusts parallel to the running direction, duringthe acceleration and breaking steps of the vehicle.

The circumferential and transverse grooves can also affect waterdraining in the contact area with the road surface (footprint area)during running on a wet road surface.

WO 2006-007877 in the name of the same Applicant, proposes a tyre thetread band of which has at least one first and one secondcircumferential groove separating a central portion from two shoulderportions. A circumferential cut is formed some distance from at leastone of the circumferential grooves.

The tread band is crossed by transverse grooves distributed according tocircumferentially repeated modules, each of which in at least oneshoulder portion has a main groove with a first substantiallyrectilinear segment inclined at an angle included between 3° and 10°relative to a radial plane of the tyre, a second substantiallyrectilinear segment extending between the circumferential cut and thefirst circumferential groove at an angle just as an indication includedbetween 105° and 130° relative to the radial plane, and a curvilinearconnecting segment between the first and second segments.

SUMMARY OF THE INVENTION

The Applicant has noticed that a high number of transverse grooves ofimportant width improves traction above all on wet road surfaces, andensures good flexibility to the block itself, but an excessive use ofsame can impair performance on dry road surfaces and increase the tyrenoise. In fact, one of the main noise causes is the continuoussuccession of impacts of the block corners on the road surface.

The Applicant has further noticed that the transverse grooves with animportant width tend to structurally weaken the tread band, which willimpair the easy drive qualities which are fundamental in a tyre alsoconceived for UHP (Ultra High Performance) uses.

It is the Applicant's firm belief that these easy drive qualities areparticularly affected by the tread pattern structure in the regionsclose to the shoulder, i.e. the axially external portion of the treadband.

In fact, the Applicant could observe that stresses transmitted betweentyre and road surface on getting over bends at high speed are morestrongly felt at these outer shoulder regions.

In order to succeed in keeping a high safety level in all useconditions, particularly for High Performance cars and SUV's, the tyremust further have excellent braking qualities (both on dry and wetsurfaces), but also be resistant to the aquaplane phenomenon; these twofeatures oppose each other because a low hollow/solid ratio is requiredas regards braking, but a suitable number of grooves and above all asuitable width of the grooves is needed in order to ensure a good waterdraining.

The Applicant has further noticed that usually the depth of thetransverse grooves in known tyres intended for high performance cars isgenerally reduced in summer tyres, as compared with winter tyres forexample. On the other hand, it is the Applicant's firm belief that alsofor summer tyres the depth of the transverse grooves cannot be reducedtoo much, because it is directly connected with the resistance to theaquaplane and the long life of the tyre.

The Applicant has found that the above described mutually conflictingproblems are solved by a tread pattern with a reduced hollow/solidratio, comprising transverse grooves of reduced width and having acourse adapted to ensure optimal traction/braking features both on astraight stretch and on a bend. In addition, the pattern can comprise agreater concentration of grooves dedicated to water discharge from thefootprint area, in the axially intermediate and/or internal regions ofthe tyre.

In more detail, in one aspect the present invention relates to apneumatic tyre for car having a tread comprising a central portiondisposed astride an equatorial plane and two shoulder portions, thecentral portion being separated from the shoulder portions of the treadby two circumferential grooves, in the central portion at least onecircumferential row included between two circumferential grooves beingpresent, characterised in that said tread has a hollow/solid ratio lowerthan 0.28; said circumferential row comprises transverse groovesextending over at least 80% of the width of said circumferential row;each transverse groove comprising a median line having at least onefirst rectilinear stretch and one curvilinear stretch; said transversegrooves having a smaller width than that of the circumferential grooves.

In the present specification and in the following claims by “extension”of the transverse grooves it is intended the length of the projection ofsaid groove on a straight line perpendicular to the equatorial planecrossing the circumferential row.

The low hollow/solid ratio together with the reduced size of thetransverse grooves give a high structural consistency to the tread bandin this region, so as to ensure excellent qualities of easy drive. Inthe present specification and in the following claims, by “hollow/solid”ratio it is intended the value of the ratio measurable in the footprintarea S between the tread portions taken up by cuts and/or grooves(hollows), and therefore without physical contact with the ground, andthe extension of the footprint area S itself.

The present invention, in said aspect, can have at least one of thepreferred features hereinafter described.

Preferably, the median line has a second rectilinear stretch and thecurvilinear stretch connects the first and second rectilinear stretches.

The second rectilinear stretch has a length smaller than or equal to onethird of the length of the first stretch.

The transverse grooves have a maximum width smaller than or equal to 1.5mm.

The transverse grooves extend from a circumferential groove to theaxially adjacent one.

Said circumferential row comprises a row of blocks; each block of therow is axially delimited by a stretch of said circumferential groovesand is circumferentially confined by two consecutive transverse grooves.The reduced size of the transverse grooves enables said block row towork as a continuous surface offering an appropriate support of thetread band in the footprint area, so as to reduce the load weighing onthe blocks thus limiting wear of same and the noise phenomena resultingfrom impact on the ground during rolling.

In order to obtain optimal traction/braking qualities when getting overa straight, the connecting stretch comprises an arc of a circumferencehaving a preselected radius of curvature (R1) and defines a convexityoriented in a first circumferential direction.

In order to obtain optimal traction/braking qualities when running on abend, the first rectilinear stretch extends over at least 40% of thewidth of the block and has a preselected inclination relative to saidequatorial plane.

The block portion at the above described curvilinear stretch and at thesecond rectilinear stretch of smaller extension is that ensuring moretraction/braking efficiency to the block on getting over a straight.

On the contrary, the rectilinear stretch of greater extension gives theblock good flexibility, while at the same time ensuring braking/tractionability when the tyre is getting over a bend.

The second stretch has a direction substantially perpendicular to theequatorial plane.

In the present specification and in the following claims by “directionsubstantially perpendicular to the equatorial plane” it is intended adirection forming an angle with the equatorial plane included in therange of 75° to 105°.

The first rectilinear stretch, instead, forms an angle α relative to theequatorial plane that is smaller than or equal to 50°, preferablysmaller than or equal to 40°.

In the present specification and in the following claims, each angleadapted to indicate the inclination of a groove, is intended to becalculated from the angle defined between a plane parallel to theequatorial plane passing through the axially closest circumferentialgroove and the plane to which said groove belongs.

According to another preferred aspect, the central portion of the treadhas a second circumferential row included between two circumferentialgrooves; said circumferential row comprises transverse grooves extendingover at least 80% of the width of the circumferential row; eachtransverse groove comprising a median line having a first rectilinearstretch and one curvilinear connecting stretch; these transverse grooveshaving a smaller width than that of the circumferential grooves.

The median line of the transverse grooves of the second circumferentialrow has a second rectilinear stretch and the curvilinear stretchconnects said first and second rectilinear stretches.

Advantageously, the first and second rectilinear stretches of the secondcircumferential row have different lengths.

The first rectilinear stretch of the second circumferential row extendsover at least 40% of the width of the circumferential row and has apreselected inclination relative to the equatorial plane.

The first rectilinear stretch forms an angle β relative to theequatorial plane that is smaller than or equal to 50°; preferablysmaller than or equal to 40°.

In a further advantageous aspect, the second stretch has a preselectedinclination relative to said equatorial plane that is adapted to definean angle δ greater than 75°.

The connecting stretch of the second circumferential row comprises anarc of a circumference having a preselected radius of curvature (R2) anddefines a concavity oriented in a second circumferential direction (M).

Said second circumferential direction is opposite to the firstcircumferential direction.

Advantageously, the transverse grooves of the second row have a maximumwidth smaller than or equal to 1.5 mm.

Preferably, the transverse grooves of the second circumferential rowextend from one circumferential groove to the axially adjacent one.

The second circumferential row comprises a block row; each block of thesecond row is axially delimited by a stretch of the circumferentialgrooves and is circumferentially delimited by two consecutive transversegrooves.

In this case too, the block portion at the curvilinear stretch and atthe second stretch substantially perpendicular to the equatorial plane,is that ensuring more traction/braking efficiency to the block ongetting over a straight.

The first rectilinear stretch, i.e. the one of greater extension, on thecontrary, gives the block good flexibility, at the same time ensuringbraking/traction ability when the tyre is running on a bend.

Preferably, the width of said transverse grooves of the first and secondrows is smaller than or equal to 1 mm.

In a still further preferred aspect, the central portion of the treadcomprises at least one third circumferential row of blocks includedbetween two circumferential grooves, each of said blocks being axiallydefined by a stretch of the circumferential grooves and beingcircumferentially defined by transverse grooves; the transverse groovescomprising a median line of substantially rectilinear course adapted toform an angle φ with the equatorial plane.

The transverse grooves of the third row extend from one circumferentialgroove to the axially adjacent one with a width greater than 1.5 mm,preferably smaller than 4 mm.

Preferably, the third row of blocks comprises a further groove adaptedto divide the block into two sub-blocks; the further groove comprises amedian line having a first and a second rectilinear stretch and acurvilinear connecting stretch joining the first and second rectilinearstretches together; the further groove having a width smaller than 1.5mm.

Preferably, the circumferential grooves have a width included in therange of 5 to 16 mm, inclusive of the extremes.

The circumferential grooves have a depth included in the range of 5 to11 mm.

The circumferential groove disposed at the axially external portion ofthe central portion (L1) of the tread has a smaller width than that ofthe circumferential grooves.

The smaller width of the axially external circumferential groovetogether with the preselected hollow/solid ratio of the axially externalportion allows the tread to offer more support surface in this regionwhich is particularly stressed on a bend, so as to increase the easydrive of the tyre.

Advantageously, the transverse grooves of the first and secondcircumferential rows have a depth reduction in a portion adjacent to atleast one circumferential groove. This step-wise course of thetransverse grooves makes the block rigid in the axial direction andreduces possible uneven-wear problems at the block corners.

The equatorial plane divides the tread into two half-regions, i.e. anaxially internal half-region (L2) and an axially external half-region(L3) positioned on the outer side of said car when said tyre is mountedon the latter. The axially external half-region (L3) has a hollow/solidratio lower than that of the axially internal half-region (L2). Inparticular, the axially internal half-region has a hollow/solid ratiolower than or equal to 0.29.

Preferably, the axially internal block row of the central portion (L1)of the tread has a hollow/solid ratio included in the range of 0.28 to0.38.

According to another preferred embodiment, each shoulder portion isformed of at least one block row, each block of the row beingcircumferentially delimited by transverse grooves.

The transverse grooves have a median line of substantially rectilinearcourse, which is inclined relative to the equatorial plane so as to forman angle ω greater than 70° with the circumferential direction of theaxially adjacent groove.

The tyre of the invention has a high grip on a wet road surface, verylow noise values, high comfort level and optimal running behaviour on adry road surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Characteristics and advantages of the invention will be now illustratedwith reference to embodiments shown by way of non-limiting example inthe accompanying figures, in which:

FIG. 1 is a perspective view of a pneumatic tyre having a treadmanufactured according to an example of the invention;

FIG. 2 is a plan view of the tyre tread seen in FIG. 1;

FIG. 2 a is a plane view of the tyre tread seen in FIG. 1;

FIG. 3 is a plan view of a variant of the tread in FIG. 2;

FIG. 4 is a plan view of an alternative embodiment of the tread in FIG.2;

FIG. 5 is a plan view of a further alternative embodiment of the treadin FIG. 2; and

FIG. 6 shows the noise spectra of a tyre of the invention and areference tyre at about 80 km/hour.

DETAILED DESCRIPTION OF THE INVENTION

A tyre 1 having a first embodiment of the tread 2 in accordance with theinvention is shown in FIGS. 1, 2, 2 b.

The structure of tyre 1 is of conventional type and comprises a carcass,a tread band placed on the carcass crown, a pair of axially oppositesidewalls, terminating with beads reinforced with bead cores andrespective bead fillers. The tyre preferably also comprises a beltstructure interposed between carcass and tread band. The carcass isreinforced with one or more carcass plies anchored to the bead cores,while the belt structure comprises two belt strips radially overlappingone another. The belt strips are formed with rubberised fabric lengthsincorporating metallic cords parallel to each other in each strip andcrossed with the cords of the adjacent strips, preferably inclined in asymmetric manner relative to the equatorial plane. Preferably, the beltstructure also comprises a third belt strip, at a radially outermostposition, provided with cords oriented substantially parallel to theequatorial plane. The cords of the zero-degree belt are preferablytextile cords and more preferably are made of a heat-shrinkablematerial. Tyre 1 preferably has a H/C ratio between the height of theright section and the maximum width of the section, that is includedbetween 0.20 and 0.65.

Tread 2 has a pattern of the asymmetric type, i.e. it operates in a moreefficient manner when tyre 1 is mounted on the car with a givenorientation rather than with the opposite one. In other words, the tyrepreferably has an inner sidewall (car side) and an outer sidewall.

To ensure the tyre a long life (kilometres run) and at the same timehigh performance over all its life, in particular as regards easy drive,the tread 2 has a reduced hollow/solid ratio, i.e. lower than 0.28,preferably lower than 0.27 and equal to about 0.25, for example.

Tread 2 is provided with circumferential grooves 3, 4, 5 and 6 (FIG. 3),extending in longitudinal direction and parallel to the equatorial plane7 of the tyre.

Tread 2 comprises a central portion L1 and two shoulder portions 8, 12.The central portion L1 has three circumferential rows 9, 10, 11, acentral one and two side rows, 9 and 11. The shoulder portion 8 isseparated from row 9 by the circumferential groove 3. Row 9 is includedbetween the circumferential grooves 3 and 4. Row 10 is included betweenthe circumferential grooves 4 and 5. Row 11 is included between thecircumferential grooves 5 and 6. The shoulder portion 12 is separatedfrom the block row 11 by groove 6.

The circumferential grooves 3, 4, 5 and 6 have a width ranging fromabout 5 mm to about 16 mm. The circumferential grooves 3, 4, 5 and 6have a depth ranging from about 5 mm to about 11 mm.

Preferably, the axially outermost circumferential groove 3 of the treadhas a smaller width than that of grooves 4, 5, 6, so as to allow tread 2to offer more support surface when running on a bend, thus increasinghandling of the tyre.

In detail, the width of the circumferential groove 3 can be included inthe range between 5 and 10.5 mm. The circumferential grooves 4 and 5,instead, are those having the greatest width, included between 9 and 16mm. The axially innermost circumferential groove 6 can, instead, have anintermediate width between 7 and 11 mm. The circumferential groove 3 canhave a depth smaller than 10 mm, preferably greater than 5 mm, equal to8 mm for example.

Alternatively, all the circumferential grooves could have the same widthand/or depth without departing from the protection scope of the presentinvention.

As shown in FIG. 2 b, the side wall 308 of the circumferential groovehas an inclination of about 20° relative to its mid-line axis, while theopposite wall 312 has an inclination of about 5° relative to themid-line axis of the same groove 3.

The circumferential groove 4 has a depth smaller than 10 mm, preferablygreater than 5 mm, more preferably equal to 8.5 mm. The side wall 408 ofthe circumferential groove 4 has an inclination of about 10° relative toits mid-line axis, while the opposite sidewall 412 can have aninclination of about 5° relative to the mid-line axis.

The circumferential groove 5 can have the same depth as groove 4 andside walls 508, 512 with inclinations that are symmetrical mirror imagesof each other.

In detail, the side wall 508, as well as side wall 512, of thecircumferential groove 5 have an inclination of about 5° relative totheir mid-line axes.

Finally, the circumferential groove 6 can have a depth smaller than 10mm, preferably greater than 5 mm, equal to 8 mm, for example. The sidewall 608 of the circumferential groove 6 has an inclination of about 5°relative to its mid-line axis. Wall 612 has the same inclination ofabout 5° relative to the mid-line axis.

The side walls of grooves 3, 4, 5, 6 could have different inclinationsrelative to their mid-line axes as compared with those mentioned abovewithout departing from the protection scope of the present invention.

As previously mentioned, the circumferential grooves 3, 6 separate thecentral portion L1 of the tread from the shoulder portions 8, 12, whilethe circumferential grooves 4, 5 divide the central portion L1 of thetread into circumferential rows 9, 10, 11.

The equatorial plane 7 divides tread 2 into two half-regions, an axiallyinternal half-region L2 and an axially external half-region L3,positioned on the outer side of a car when tyre 1 is mounted on saidcar.

The axially external half-region L3 has a hollow/solid ratio greaterthan that of the axially internal half-region L2. The hollow/solid ratioof half-region L2 is lower than or equal to 0.29.

At least one of the circumferential rows 9, 10, 11 comprisescircumferential grooves extending over at least 80% of the row width.Preferably, the transverse grooves extend over the whole width of therow so as to define blocks.

In detail, row 9 includes a series of blocks 13, row 10 includes aseries of blocks 14 and row 11 includes a series of blocks 15. Tread 2(FIG. 2) has a hollow/solid ratio of about 0.27 in the circumferentialblock row 9; a hollow/solid ratio of about 0.3 in the circumferentialblock row 10; a hollow/solid ratio of about 0.36 in the circumferentialblock row 11; and a hollow/solid ratio of about 0.2 in the shoulderportion 12. The differentiated hollow/solid ratio on the two sides, i.e.lower on the outer side and higher on the inner side of the tyre,assists behaviour of the car running on a dry road surface, above allwhen a High Performance car is concerned in which greater camber anglesare adopted that make the footprint of the casing (tread) asymmetric.

Each block 13 of the circumferential row 9 is axially delimited by twostretches of circumferential grooves 103 and 104 and circumferentiallydelimited by two transverse grooves 16.

In the preferred embodiment shown in FIGS. 1-2 b, each transverse groove16 extends from the axially outermost circumferential groove 3 to theadjacent circumferential groove 4. Each transverse groove 16 has amedian line provided with at least one first rectilinear stretch 107 andone curvilinear connecting stretch 108.

The median lines of two transverse grooves 16 whatever, that arecircumferentially consecutive, have a parallel course at least in aportion of their extension. Preferably, said median lines have aparallel course over the whole extension thereof.

Still in the preferred embodiment shown in the above mentioned FIGS. 1-2b, the median lines of the transverse grooves 16 have a secondrectilinear stretch 106 and the curvilinear stretch 108 is suchpositioned as to join the first 107 and second 106 rectilinear stretchestogether.

The second rectilinear stretch 106 has a length smaller than or equal toone third of the length of the first stretch 107.

The curvilinear stretch 108 is defined by an arc of a circumferencehaving a preselected radius of curvature R1 included in the range of 5to 15 mm, preferably of 9 to 12 mm.

The curvilinear connecting stretch 108 joins the first 107 and second106 rectilinear stretches together so as to define a convexity orientedin a first circumferential direction, denoted by arrow F in FIG. 2. Thefirst circumferential direction is opposite to the rolling direction ofthe tyre denoted by arrow A in FIG. 2.

The second rectilinear stretch 106 is substantially perpendicular to theequatorial plane 7. In other words, the second rectilinear stretch 106forms an angle θ with the circumferential direction of thecircumferential groove that is included between 75° and 105°.

The portion of block 13 at the curvilinear stretch 108 and at the secondrectilinear stretch 106 is that ensuring more traction/braking to theblock when the tyre is getting over a straight.

The first rectilinear stretch 107 is, instead, disposed in an inclineddirection relative to the equatorial plane 7, so that it is lessinclined as compared with the equatorial plane 7 of the firstrectilinear stretch 106.

In particular, the first rectilinear stretch 107 forms an angle αrelative to the direction of the circumferential groove 4 that issmaller than 50°. Preferably angle α is smaller than 40°. Preferably,the second rectilinear stretch 107 forms an angle α relative to thedirection of the circumferential groove 4, that is greater than 20°,more preferably greater than 23°.

The first rectilinear stretch 107 has an extension greater than 40% ofthe width of block 13.

Such an arrangement and extension of the first rectilinear stretch 107gives block 13 an optimal flexibility, at the same time ensuringbraking/traction ability when the tyre is running on a bend.

The transverse grooves 16 have a constant width moving from the axiallyexternal circumferential groove 3 to the axially adjacentcircumferential groove 4. The transverse grooves 16 have a smaller widththan that of the circumferential grooves 3, 4, 5, 6. In detail, thetransverse grooves 16 have a width smaller than 1.5 mm, preferablysmaller than 1 mm, more preferably smaller than or equal to 0.8 mm.

Such a reduced size of grooves 16 ensures a high amount of “rubber onthe ground” at the row 9 of blocks 13, therefore a low hollow/solidratio and, as a result, excellent handling qualities and low noise.

Grooves 16, as shown in FIG. 2 b, further have a decreasing depth fromthe axially external groove 3 to the adjacent circumferential groove 4.In particular, the transverse grooves 16 have a depth with a step-wisecourse. In other words, as shown in chain line in FIG. 2 b, they have aregion of reduced depth 38 close to the axially innermostcircumferential groove 4.

Depth of grooves 16 is preferably less than 9 mm. At the portion withreduced depth 38, placed close to the circumferential groove 4, grooves16 have a depth less than 3 mm, of 2 mm for example.

This step-wise course of grooves 16 gives rigidity to block 13 at asharp corner thereof so as to eliminate or at all events reduce theuneven wear typically concerning these portions.

In any case, grooves 16 could have a constant width without departingfrom the protection scope of the present invention.

To further reduce the possibility that an uneven wear of the block mayinitiate and consequently noise problems connected with this type ofwear may take place, at the sharp corners of the block, grooves 16 havea bevel 78 close to the circumferential groove 4.

Adjacent to the circumferential groove 4, still as shown in FIG. 2,there is the row 10 of blocks 14.

The row 10 of blocks 14 is obtained through a 180° rotation of thepattern of the row 9 of blocks 13 relative to the circumferential groove4. In detail, the block row 10 is disposed substantially astride theequatorial plane 7 and axially extends over at least 20%, but less than40%, of the central portion L1.

Each block 14 of row 10 is axially delimited by two stretches ofcircumferential grooves 204, 205 that are axially spaced, andcircumferentially delimited by two transverse grooves 26 that arecircumferentially spaced and extend over at least 80% of the width ofthe circumferential row 10.

In the preferred embodiment shown in FIGS. 1-2 b, each transverse groove26 extends from the axially outermost circumferential groove 4 to theadjacent circumferential groove 5. Each transverse groove 26 has amedian line provided with at least one first rectilinear stretch 207 andone curvilinear connecting stretch 208.

The median lines of two circumferentially consecutive transverse grooveswhatever 26, have a parallel course at least on a portion of theirextension. Preferably, said median lines have a parallel course over thewhole extension thereof.

Still in the preferred embodiment shown in the above mentioned FIGS. 1-2b, the median line of the transverse grooves 26 has a second rectilinearstretch 206 and the curvilinear stretch 208 is such positioned as tojoin the first 207 and second 206 rectilinear stretches together. Thefirst 207 and second 206 rectilinear stretches of the secondcircumferential row 10 have different lengths.

The curvilinear stretch 208 of each groove 26 is defined by an arc of acircumference having a preselected radius of curvature R2 included inthe range of 5 to 15 mm, preferably of 9 to 12 mm.

The curvilinear connecting stretch 208 joins the first 207 and second206 rectilinear stretches together, so as to define a convexity orientedin a second circumferential direction, denoted by arrow M in the figureand opposite to the first circumferential direction F.

In this case too the portion of block 14 at the curvilinear stretch andat the second rectilinear stretch described above is that ensuring moretraction/braking to the tyre running on a straight.

The first rectilinear stretch 207 is disposed in an inclined directionrelative to the equatorial plane 7 and is less inclined than theequatorial plane of the second rectilinear stretch 206.

In particular, the first rectilinear stretch 207 forms an angle βsmaller than 50°, preferably smaller than 40°, relative to the directionof the circumferential groove. Preferably, the first rectilinear stretch207 forms an angle β greater than 20°, more preferably greater than 23°,relative to the direction of the circumferential groove 4. The firstrectilinear stretch 207 extends over at least 40% of the width of block14.

Such an arrangement and extension of the first rectilinear stretch 207gives the block 14 an optimal flexibility, at the same time ensuringbraking/traction ability when the tyre is running on a bend.

The second rectilinear stretch 206 is substantially perpendicular to theequatorial plane 7. In other words, the second rectilinear stretch 207forms an angle δ with the circumferential direction of thecircumferential groove 7 which is included between 75° and 105°.

The transverse grooves 26 have a constant width moving from the axiallyoutermost circumferential groove 4 to the axially adjacentcircumferential groove 5. The transverse grooves 26 have a smaller widththan that of the circumferential grooves 3, 4, 5, 6. In detail, thetransverse grooves 26 have a width smaller than 1.5 mm, preferablysmaller than 1 mm, more preferably smaller than or equal to 0.8 mm.

Each groove 26 has a decreasing depth starting from grooves 4, 5 towardsits central region. In particular, the transverse grooves 20 have adouble-step course with reduced depth so as to form two raised portions45, towards the circumferential grooves 4 and 5.

The depth of grooves 26 of row 10 in the central region is constant andsmaller than 9 mm, preferably equal to 7 mm. At the raised portions 45,placed close to the circumferential grooves 4 and 5, grooves 26 have adepth smaller than 3 mm, preferably equal to 2 mm. This course ofgrooves 26 gives rigidity to block 14 in an axial direction.

At all events, grooves 26 could have a constant depth over the wholeextension thereof without departing from the protection scope of thepresent invention.

To reduce the possibility that an uneven wear of the block may initiateand consequently noise problems connected with this type of wear mayarise, at the sharp corners of block 14, a bevel 79 is present.

Adjacent to the circumferential groove 5 there is the block row 11. Indetail, the block row 11 is axially spaced towards the inside relativeto the equatorial plane 7.

Each block of said row 11 is axially delimited by two stretches ofcircumferential grooves 305, 306 that are axially spaced and iscircumferentially delimited by two transverse grooves 36 that arecircumferentially spaced.

Each transverse groove 36 extends from the axially innermostcircumferential groove 5 to the adjacent circumferential groove 6 andhas a median line having a substantially rectilinear course.

The median lines of the transverse grooves 36 that are circumferentiallyconsecutive and therefore adapted to define a block 15 have asubstantially parallel course over at least one portion of theirextension. Preferably, said median lines have a parallel course over thewhole extension thereof.

The median line of each groove 36 is disposed in an inclined directionrelative to the equatorial plane 7. In particular, the median line ofeach groove 36 forms an angle φ relative to the direction of thecircumferential groove 5 smaller than 95°, preferably smaller than 90°.

Preferably, the median line of each groove 36 forms an angle φ relativeto the direction of the circumferential groove 5 greater than 70°, morepreferably greater than 75°.

The transverse grooves 36 separating blocks 15 of rows 11 substantiallyhave a constant width moving from the circumferential groove 5 to theaxially adjacent circumferential groove 6. The transverse grooves 36have a smaller width than that of the circumferential grooves 3, 4, 5,6, but at all events greater than that of the transverse grooves 16 and26. In detail, the transverse grooves 36 have a width less than 4 mm,preferably less than 3 mm. Preferably, grooves 36 in any case have awidth greater than 1.5 mm, preferably greater than 2 mm.

Given the size of the transverse grooves 36 and the circumferentialgrooves 5, 6, and consequently the hollow/solid ratio that is higher inthis region than in the rest of the tyre, this region allows optimalwater draining and more resistance to the aquaplane phenomenon.

To give tyres 15 more structural stability, in favour of easy drive,quietness on running and wear evenness, blocks 15 can be connected witheach other by reinforcing elements 31 located in grooves 36. In moredetail, taking into account the longitudinal section of a groove 36, asshown in FIG. 2 b, each reinforcing element 31 can be defined by aportion of reduced depth provided at the central region of therespective transverse groove 36. Depth of the transverse groove 36 atthe central region, i.e. of element 31, can be just as an indicationincluded between 1.5 mm and 6 mm, and be preferably equal to 4 mm.

In the remaining portion, groove 36 has a varying depth increasing fromgrooves 5 and 6 towards the reinforcing element 31. In detail, close tothe circumferential grooves 5 and 6, groove 36 has a depth includedbetween 6 and 9 mm, preferably equal to 8 mm.

Each block 15 is further divided into two sub-blocks 15′, 15″ by afurther groove 46.

Sub-block 15″ substantially has an L-shaped conformation, whilesub-block 15′ enclosed by the two arms forming the L conformationsubstantially has a trapezium shape.

Each groove 46 extends from the circumferential groove to the transversegroove 36 and has a median line with a first rectilinear stretch 406, asecond rectilinear stretch 407 and a curvilinear connecting stretch 408joining the first 406 and second 407 rectilinear stretches together.

The curvilinear stretch 408 of each groove 46 is defined by an arc of acircumference having a preselected radius of curvature R3 equal to theradius of curvature R1.

The first rectilinear stretch 406 has a course substantially parallel tothat of the transverse grooves 36, while the second rectilinear stretch407 forms an angle γ with a direction substantially parallel to theequatorial plane of a value higher than 5°, preferably lower than 20°,more preferably equal to 10°. The second rectilinear stretch 407 extendsuntil a transverse groove 36 and joins the latter axially downstream ofthe reinforcing element 31.

Each groove 46 has a decreasing depth from grooves 5, to its centralregion. In particular, as better shown in FIG. 2 b, the transversegrooves 46 have a double-step course with reduced depth towards grooves5 and 36.

Depth of grooves 46 is lower than 9 mm, preferably equal to 7 mm. At theraised portions 35 or steps placed close to grooves 5 and 36, grooves 46have a depth lower than 3 mm, preferably equal to 2 mm.

This step-wise course of grooves 46 gives rigidity to block 15 andreduces possible problems of uneven wear.

Grooves 46 have a width smaller than 1.5 mm.

Grooves 46 can continue into groove 5 for connection with grooves 26 soas to form a single groove, as shown in the embodiment in FIG. 3, forexample.

As previously mentioned, the two shoulder portions 8 and 12 are axiallylimited relative to the central portion L1 of tread 2 by grooves 3 and6, respectively.

Each shoulder portion 8 and 12 comprises transverse grooves 56 and 66,respectively.

The transverse grooves 56 and 66 repeat themselves circumferentially.The transverse grooves 56 and 66 have a median line with a substantiallyrectilinear course, substantially perpendicular to the equatorial plane7.

In particular, the median line of grooves 56 form an angle ω greaterthan 70° with the circumferential direction of groove 3. In theembodiment shown in the figures said angle is smaller than 90° andpreferably corresponds to 80°.

The median line of grooves 66 too is substantially oriented as themedian line of grooves 56, forming therefore an angle ω greater than70°. In the embodiment shown in the figures said angle ω is smaller than90° and preferably corresponds to 80°.

Grooves 56 of the axially external shoulder 8 extend from the axiallyexternal edge of the tread band 2 until close to groove 3, withouthowever reaching it.

On the contrary, grooves 66 of the axially internal shoulder 12 extendfrom the axially internal edge of the tread band 2 to groove 6. Indetail, in the last stretch towards the circumferential groove 6, eachtransverse groove 66 has a section narrowing and passes from a widthgreater than 2 mm to a width smaller than 1.5 mm, preferably equal to0.8 mm.

The transverse grooves 66 and grooves 56 in the stretch not submitted tonarrowing have a width greater than 2 mm, preferably greater than 2.5mm. Preferably, grooves 56, 66 have a width smaller than 4 mm.

Grooves 56, 66 do not have a constant depth, but a varying depth thatdecreases on moving towards the axially external edge of the tread band2 as regards the circumferential groove 56 and towards the axiallyinternal edge of the tread band 2 as regards the circumferential groove66. The consecutive transverse grooves 56 as well as the consecutivegrooves 66 are joined two by two by substantially longitudinal grooves96, 86 having an S-shaped course. The longitudinal grooves 96, 86 have amedian line with three right portions and two curvilinear portions.

Each curvilinear portion is disposed so as to join two rectilinearportions. Each longitudinal groove 96, 86 substantially extends from theaxially external edge of tread 2 to the axially adjacent circumferentialgroove. In particular, each longitudinal groove 96 extends from theaxially external edge of tread 2 until close to the axially internal endof the transverse groove 56.

Each longitudinal groove 86 substantially extends from the axiallyinternal edge of the tread band 2 until close to the axially internalend of the transverse groove 66. In particular, the longitudinal groove86 extends until close to the inner edge of the groove 66 of greaterwidth.

In order to increase the structural rigidity of the shoulder blocks, thelongitudinal grooves 96, 86 have a varying depth that decreases at theends forming steps.

In particular, each longitudinal groove 96, 86 has a substantiallyconstant depth which is more than 3 mm and less than 8 mm, preferablyequal to 7 mm in the rectilinear stretch substantially perpendicular tothe equatorial plane.

In the curvilinear connecting stretches and in the remainingsubstantially circumferential rectilinear stretches characterised by adepth reduction, the longitudinal grooves 96, 86 have a depth includedbetween 1 mm and 5 mm, preferably equal to 2 mm.

However, the longitudinal grooves 96, 86 could have different depthswithout departing from the protection scope of the present invention.

In tread 2 the shoulder portion 8 has a hollow/solid ratio lower thanthat of the shoulder portion 12.

In FIG. 3 a tread 102 is shown that is a variant of that seen in FIG. 2and in which the same parts are identified with same numbers. Tread 102is quite similar to tread 2 except for the transverse grooves 56 of theouter shoulder portion 8. In this case, in fact, the transverse grooves56 extend from the axially external edge of tread 102 to thecircumferential groove 3, and the shoulder portion 8, like the shoulderportion 12, in this case has the same hollow/solid ratio.

In FIG. 3 the running direction is denoted by arrow A.

Shown in FIG. 4 is a tread 112 that is a further variant of that seen inFIG. 2 and in which the same parts are identified with same numbers.

In this case, tread 112 is provided with three circumferential grooves3, 4, 5 (FIG. 4) extending in longitudinal direction and being parallelto an equatorial plane 7 of the tyre. Tread 112 includes twocircumferential central rows 9, 10 of blocks. The shoulder portion 8 isseparated from the block row 9 by the circumferential groove 3. The row9 of blocks 13 is included between the circumferential grooves 3 and 4.The row 10 of blocks 14 is included between the circumferential grooves4 and 5. The shoulder portion 12 is separated from the row 10 of blocks14 by groove 5.

The circumferential grooves 3, 4 and 5 all substantially have the samewidth, included between 9 and 14 mm.

The row of blocks 13 of tread 112 corresponds in shape and position tothe homologous row in FIG. 2 and for this reason will not be furtherdescribed.

As regards, instead, the row 10 of blocks 14, this block row correspondsin shape to the homologous block row in FIG. 2 but has a differentposition. In particular, the block row 10 seen in FIG. 4 is axiallyspaced relative to the equatorial plane 7.

Finally, shown in FIG. 5 is a tread 122 that is a variant of the oneseen in FIG. 2 and in which the same parts are identified with the samereference numerals.

In particular, tread 122 in FIG. 5 is quite similar to the tread in FIG.4 except that the row 10 of blocks 14 has been replaced by the row 11 ofblocks 15.

Row 11 of blocks 15 corresponds in shape to the homologous block row inFIG. 2 and for this reason will not be further described.

The row 11 of blocks 15 shown in FIG. 5 is only axially displaced to amore centred position. In this way, relative to the tread in FIG. 4, thetread seen in FIG. 5 has a differentiated hollow/solid ratio on the twosides, i.e. a lower ratio on the outer side and a higher ratio on theinner side.

A specimen of the tyre of the invention having the tread 2 shown inFIGS. 1-2 was manufactured and submitted to comparison tests with acontrol tyre P having a tread with a central portion separated from twocircumferential grooves by two shoulder portions. Said central portionhas a row of central blocks and two side ridges provided with transversegrooves of curvilinear extension.

The control tyre P was selected because it has optimal features and washomologated for fast High Performance sports cars.

The tyre of the invention had size 225/50 R17, with rim 7.5×19J andinflating pressure of 2.2 bars. The control tyre had the same size.

A car Audi A6 was first equipped with four pneumatic tyres of theinvention and then with four pneumatic tyres for comparison (controltyres).

Aquaplane tests were carried out on a straight and on a bend, as well asbraking tests on a dry and wet road surface, behaviour tests duringrunning on a wet and dry road surface, noise tests at the inside andoutside of the car and comfort tests.

The aquaplane test on a straight took place on a straight stretch ofsmooth asphalt of a predetermined length (100 m) with a water layer ofpredetermined constant height (7 mm) which was automatically restoredafter each passage of the tested car. The straight is taken at constantspeed (about 70 km/hour) under full-grip conditions and then the car isaccelerated until loss of the full-grip conditions.

The aquaplane test on a bend took place on a travel stretch with smoothdry asphalt at a bend with a constant radius (100 m) having apredetermined length and comprising, at a final stretch, a region ofpredetermined length (20 m) flooded with a water layer of predeterminedthickness (6 mm). The test took place at constant speed for differentspeed values.

During the test, maximum centrifugal acceleration and maximum speed ofthe car corresponding to the complete-aquaplane condition were measured.

The braking test was carried out on a straight asphalt stretch, bothunder wet and dry ground conditions, detecting the stop distance from apredetermined starting speed, typically 100 km/h under dry conditionsand 80 km/h under wet conditions. The stop distance is determined as thearithmetic mean of a series of subsequent detecting operations.

The behaviour test on running under conditions of dry and wet surfacestakes place along predetermined paths of travel, typically circuitsclosed to traffic.

Through simulation of some characteristic manoeuvres (such as change oflane, Overtaking, slalom between traffic cones, getting in and out ofbends) carried out at constant speed, as well as while accelerating anddecelerating, evaluation of the tyre performance is carried out by thetest driver who gives a numeric evaluation of the concerned tyre duringthe above described manoeuvres.

The assessment scale represents a subjective judgement expressed by thetest driver who tests and compares the equipment items in sequence.

Comfort was evaluation taking into account the assembly of sensationsfelt by the test driver relative to the tyre capability of absorbing theroadway unevennesses.

The test results are reproduced in Table I where the assessment valuesare set out in percentage, considering the control tyre's values equalto 100.

TABLE I Control Invention Aquaplane on a bend 100 100 Aquaplane on astraight 100 100 Braking on a dry ground 100 107 Braking on a wet ground100 102 Behaviour on a dry ground 100 105 Behaviour on a wet ground 100102

In Table I, values greater than 1 denote an improvement as compared withthe control tyre.

The test results prove that the tyre of the invention has a quite betterbehaviour than the control tyre in the braking tests and behaviour testson a dry ground.

Shown in FIG. 6 are the graphs of the outside noise intensity dB(A)depending on the frequency, for the tyre of the invention (curve B) andthe control tyre (curve A). As known, in these tests the reference speedis 80 km/h.

The graphs in FIG. 6 show that the tyre of the invention appears to beless noisy than the control tyre by about 2 dB(A) on an average.

The invention claimed is:
 1. A pneumatic tyre comprising a treadcomprising a central portion disposed astride an equatorial plane andtwo shoulder portions, the central portion being separated from theshoulder portions of the tread by two circumferential grooves; saidcentral portion comprising at least one circumferential row between twocircumferential grooves wherein said tread has a hollow/solid ratio lessthan 0.28; said circumferential row comprises transverse groovesextending over at least 80% of the width of said circumferential row;each transverse groove comprises a median line having at least one firstrectilinear stretch and one curvilinear stretch; said transverse grooveshaving a smaller width than that of the circumferential grooves; whereinthe equatorial plane divides the tread into two half-regions, an axiallyinternal half-region and an axially external half-region positioned onan outer side of a car, when said tyre is mounted thereon, said axiallyexternal half-region has a hollow/solid ratio less than that of theaxially internal half-region, and said axially internal half-region hasa hollow/solid ratio less than or equal to 0.28.
 2. The pneumatic tyreas claimed in claim 1, wherein said median line has a second rectilinearstretch and said curvilinear stretch connects said first and a secondrectilinear stretches.
 3. The pneumatic tyre as claimed in claim 2,wherein said second rectilinear stretch has a length less than, or equalto one third of the length of the first stretch.
 4. The pneumatic tyreas claimed in claim 1, wherein said transverse grooves have a maximumwidth less than or equal to 1.5 mm.
 5. The pneumatic tyre as claimed inclaim 1, wherein said transverse grooves extend from a circumferentialgroove to an axially adjacent groove.
 6. The pneumatic tyre as claimedin claim 1, wherein said circumferential row comprises a row of blocks,each block of the row being axially delimited by a stretch of saidcircumferential grooves and being circumferentially confined by twoconsecutive transverse grooves.
 7. The pneumatic tyre as claimed inclaim 2, wherein a connecting stretch comprises an arc of acircumference having a preselected radius of curvature and defines aconvexity oriented in a first circumferential direction.
 8. Thepneumatic tyre as claimed in claim 1, wherein said first stretch extendsover at least 40% of the width of the circumferential row and has apreselected inclination relative to said equatorial plane.
 9. Thepneumatic tyre as claimed in claim 2, wherein said second stretch has adirection substantially perpendicular to said equatorial plane.
 10. Thepneumatic tyre as claimed in claim 9, wherein said first stretch formsan angle relative to said equatorial plane that is less than or equal to50°.
 11. The pneumatic tyre as claimed in claim 1, comprising at leastone second circumferential row between two circumferential grooves; saidcircumferential row comprising transverse grooves extending over atleast 80% of the width of said circumferential row; each transversegroove comprising a median line having a first rectilinear stretch andone curvilinear connecting stretch; said transverse grooves having asmaller width than the width of said circumferential grooves.
 12. Thepneumatic tyre as claimed in claim 11, wherein said median line of thetransverse grooves of the second circumferential row has a secondrectilinear stretch, and in that said curvilinear stretch connects saidfirst and a second rectilinear stretches.
 13. The pneumatic tyre asclaimed in claim 12, wherein said first and second rectilinear stretchesof the second circumferential row have different lengths.
 14. Thepneumatic tyre as claimed in claim 11, wherein said first stretchextends over at least 40% of the width of the circumferential row andhas a preselected inclination relative to said equatorial plane.
 15. Thepneumatic tyre as claimed in claim 14, wherein said first stretch formsan angle relative to said equatorial plane that is less than or equal to50°.
 16. The pneumatic tyre as claimed in claim 12, wherein said secondstretch has a preselected inclination relative to said equatorial planeadapted to define an angle greater than 75°.
 17. The pneumatic tyre asclaimed in claim 11, wherein said curvilinear connecting stretchcomprises an arc of a circumference having a preselected radius ofcurvature and defines a concavity oriented in a second circumferentialdirection.
 18. The pneumatic tyre as claimed in claim 17, wherein saidsecond circumferential direction is opposite to said firstcircumferential direction.
 19. The pneumatic tyre as claimed in claim11, wherein said transverse grooves of the second circumferential rowhave a maximum width less than or equal to 1.5 mm.
 20. The pneumatictyre as claimed in claim 11, wherein said transverse grooves of thesecond circumferential row extend from one circumferential groove to anaxially adjacent circumferential groove.
 21. The pneumatic tyre asclaimed in claim 11, wherein said second circumferential row comprises arow of blocks, each block of the second row being axially delimited by astretch of the circumferential grooves and being circumferentiallydelimited by two consecutive transverse grooves.
 22. The pneumatic tyreas claimed in claim 19, wherein the width of said transverse grooves isless than or equal to 1 mm.
 23. The pneumatic tyre as claimed in claim1, comprising at least one third circumferential row of blocks betweentwo circumferential grooves, each of said blocks being axially definedby a stretch of said circumferential grooves and being circumferentiallydefined by transverse grooves, said transverse grooves comprising amedian line of substantially rectilinear course adapted to form an anglewith said equatorial plane.
 24. The pneumatic tyre as claimed in claim23, wherein said transverse grooves extend from one circumferentialgroove to an axially adjacent circumferential groove with a widthgreater than 1.5 mm and less than 4 mm.
 25. The pneumatic tyre asclaimed in claim 23, wherein said third row of blocks comprises afurther groove capable of being adapted to divide the block into twosub-blocks; said further groove comprising a median line having a firstand a second rectilinear stretch and one curvilinear connecting stretchjoining said first and second rectilinear stretches together, saidgroove having a width less than 1.5 mm.
 26. The pneumatic tyre asclaimed in claim 1, wherein said circumferential grooves have a width of5 to 16 mm, inclusive of the extremes.
 27. The pneumatic tyre as claimedin claim 1, wherein said circumferential grooves have a depth of 5 to 11mm.
 28. The pneumatic tyre as claimed in claim 1, wherein saidcircumferential groove disposed at an axially external portion of thecentral portion of the tread has a smaller width than the width of thecircumferential grooves.
 29. The pneumatic tyre as claimed in claim 1,wherein the transverse grooves have a depth reduction in a portionadjacent to at least one circumferential groove.
 30. The pneumatic tyreas claimed in claim 1, wherein an axially internal row of blocks of thecentral portion of the tread has a hollow/solid ratio of 0.28 to 0.38.31. The pneumatic tyre as claimed in claim 1, wherein each shoulderportion comprises transverse grooves that repeat circumferentially. 32.The pneumatic tyre as claimed in claim 31, wherein the transversegrooves have a median line of substantially rectilinear course, which isinclined relative to the equatorial plane so as to form an angle greaterthan 70° with the circumferential direction of an axially adjacentgroove.